Technical Field
The present invention relates to a dynamic microphone.
Related Art
To produce an omni-directional component in a dynamic microphone, it is necessary to secure an air chamber having a sufficient length as a propagation path of a sound wave on a back side of a diaphragm. For example, a dynamic microphone used for a wireless microphone requires a space for mounting a transmitter, and thus it is difficult to secure the air chamber having a sufficient length. Therefore, among the dynamic microphones used for a wireless microphone and the like, it is difficult to obtain unidirectivity in a low frequency band.
The reason why the air chamber having a sufficient length as a passage of the sound wave is necessary on the back side of the diaphragm will be further described. Now, if there is a wall that partitions the space behind the diaphragm, in proximity to the back of the diaphragm, the air moved by vibration of the diaphragm is immediately bounced back by the wall and limits the vibration of the diaphragm, and vibration faithful to the sound wave is impeded. That is, it is in a high reactance state. Especially, an influence of the reactance is profound in a low frequency band. Therefore, to obtain the omni-directional component up to the low frequency band, it is necessary to make the path where the sound wave passes long on the back side of the diaphragm.
As a document that describes the above theory, there is “Design conditions of the damped pipe for the directional ribbon microphone”, Journal of Acoustical Society of Japan, Volume 18, No. 5 (1962). The document describes that, regarding a damping element of a directional ribbon microphone, a long and narrow acoustic tube is widely used because ribbon microphones have small mechanical impedance of vibration system. Further, the document describes that the length of the acoustic tube can be made short by making acoustic resistance of the acoustic tube higher going away from the position of the diaphragm.
There are JP 11-252675 A, JP 2015-12435 A, and JP 2015-15613 A, as patent documents related to the present invention, which is described in detail below.